U.S. Pat. No. 5,431,170 discloses a pulse rate meter comprising a sensor unit attached to a headband so that the sensor unit is applied against the forehead when the headband is being worn. The sensor unit comprises two light emitters and light sensors for receiving light from the respective emitters after reflection within the forehead body tissue. One emitter emits infra-red light, and the electrical signal provided by the sensor receiving light from this emitter will vary in accordance with the pulsations in the blood flow through the blood vessels in the forehead. The other emitter emits light of a different and preferably substantially shorter wavelength, e.g. yellow light, such that the signal from its sensor is relatively independent of variations due to the flow pulsations. Both signals do, however, vary in accordance with vibrations of the forehead due to movements. A noise cancellation circuit may carry out a frequency analysis of the output signals from the respective sensors: peaks at corresponding frequencies or frequency ranges in the two signals are regarded as noise and the remaining frequency in the signal from the infra red sensor represents the pulse rate.
A problem of the known system is that it is relatively obtrusive and only able to compensate for motion of one particular body part, namely that to which the sensor unit is attached. Movement of e.g. the arms would affect blood flow, but would not be picked up by the combination of emitter and receiver operating at the shorter wavelength, whereas it would make it more difficult to detect blood flow variations due to respiration or beating of the heart.